1. Field of the Invention
The present invention relates generally to apparatus for conditioning the air within an indoor space and, more specifically, for heating the air within an indoor space utilizing one or the other of two selectively operable sources of heat. Many residential dwellings and light commercial structures in the United States today utilize a forced air-type heating system wherein air is circulated through a furnace to be heated and thereafter discharged to the indoor space. Typically, furnaces of this type utilize a fossil fuel such as oil or natural gas which, through combustion within a heat exchanger, transfer heat to the air; in some instances, however, electrical resistance heating elements are utilized as the heat source in this type of furnace. It is well known that the supplying of heat to residential dwellings and the like represents a large percentage of the total energy consumed today in the United States, which has led to increasing emphasis on energy conserving steps which may be taken with respect to such dwellings, such as improved insulation, setback type thermostats, the government-recommended reduced indoor temperature settings during the heating season.
In structures of the type under consideration, it is also common that means are provided to effect cooling of the air circulating to the indoor space by a refrigeration unit of the vapor-compression type. Such an installation generally entails the provision of an indoor heat exchange coil disposed within the plenum of the existing furnace over which air is circulated and cooled during the cooling season.
It is further known in the art that a vapor compression refrigeration system may be reversed in order to provide heat to an indoor space through the provision of a reversing valve within the refrigeration system and associated controls therefor, such a unit being commonly referred to as a heat pump. The heat pump, although operated by electricity, has definite operating advantages over resistance-type electric heat in terms of energy efficiency, and, during certain times of the year exhibits an operating efficiency which makes its cost of operation competitive with that of a fuel-fired heat exchanger utilizing natural gas or oil. Accordingly, certain operating advantages may be derived through the provision of a heating system wherein both a selectively operable heat pump and another selectively operable heat source are provided, along with suitable control means such that the heat pump is operated at those times when it is advantageous to do so. In practice, the present invention has application not only in new construction but also may be retrofitted to existing installations wherein a furnace of the forced-air type already exists. In many cases, the heat pump may replace an existing central air conditioning unit or, the building owner may simply choose to install a heat pump in order to achieve not only improved heating efficiencies, but also to obtain the benefits of air cooling and dehumidifcation during the summer.
2. Description of the Prior Art
A prior art system of the type under consideration is disclosed in U.S. Pat. No. 3,996,998 to Garst et al. In this system, a furnace is provided having first heat transfer means such as a fuel-fired or electric heat exchanger. Located within ductwork downstream from the furnace is the indoor coil of a selectively operable heat pump. Control means are provided for selectively operating either the first heat transfer means or the heat pump in order to supply heat to an indoor space. The control means disclosed include an indoor thermostat having a first set of contacts which close in response to a relatively low demand for heat, and a second set of contacts which close in response to a relatively high demand for heat. The heat pump is operated in response to closing of said first contacts, and, upon closing of the second contacts, operation of the heat pump is terminated and the first heat transfer means is caused to operate. Once the relatively high demand for heat is satisfied, operation of the first heat transfer means is terminated and the heat pump is once again operated until such time as the relatively low demand for heat is satisfied. Time delay means are provided for preventing re-energization of the heat pump immediately following termination of operation of the first heat transfer means in order to prevent excessive refrigerant pressures within the indoor coil of the heat pump.